<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
    window.onload = function() {
        function $(id) {
            return document.getElementById(id);
        }

        $("info").innerHTML = "red cell clientWidth : " + $("td1").clientWidth +
                "<br/>blue cell clientWidth : " + $("td2").clientWidth +
                "<br/>gold cell clientWidth : " + $("td3").clientWidth;
    }
</script>
</head>
<body>
<table id="T" style="color:white;" cellpadding="0" cellspacing="0">
    <tr>
        <td id="td1" style="background-color:red;">td1</td>
        <td id="td2" style="background-color:blue;">td2</td>
    </tr>
    <tr>
        <td id="td3" style=" background-color:gold; width:300px;" colspan="2">td3</td>
    </tr>
</table>
computed clientWidth:
<div id="info" style="border:1px dashed red; width:300px;"></div>
</body>
</html>